Beamformers are used in multibeam antennas to produce output beams on the basis of radiofrequency input signals. In a known manner, there exist planar quasi-optical beamformers using electromagnetic propagation of radiofrequency waves between two parallel metal plates, in general according to a TEM (Transverse Electric Magnetic) mode of propagation for which the electric and magnetic fields are orthogonal to the direction of propagation of the radiofrequency waves. The TEM mode propagates in the parallel-plate guide at the same speed as in vacuo, thus rendering the said guide non-dispersive for this TEM mode. The focusing and collimation of the beams can be carried out by a constrained lens, as for example described in documents U.S. Pat. No. 3,170,158 and U.S. Pat. No. 5,936,588 which illustrate the case of a Rotman lens, or alternatively by a reflector as described for example in documents FR 2944153 and FR 2 986377 for Pillbox beamformers, the constrained lens, or respectively the reflector, being inserted on the propagation path of the radiofrequency waves, between the two parallel metal plates. The constrained lens, or the reflector, serves essentially as phase corrector and makes it possible, by transmission in the case of a lens, or after reflection in the case of a reflector, to convert cylindrical wavefronts into plane wavefronts.
A Pillbox beamformer can, at output, be connected to a linear array of several individual radiating elements aligned side by side. As an alternative to the use of several individual radiating elements, it is also possible to connect the linear output aperture, situated between the two parallel plates, to a single linear output horn which produces the transition between the parallel plates and the free space where the beams are radiated. In the case of the use of a single linear horn, the radiating aperture at the output of the Pillbox beamformer is linear and extends continuously over the whole transverse width of the parallel plates. These radiating linear apertures, which are not spatially quantized, have much higher performance with respect to linear arrays of several radiating elements, for beams which are squinted with respect to the focal axis, because of the absence of quantization, and exhibit a much greater bandwidth because of the absence of resonant propagation modes. However, a Pillbox beamformer exhibits the drawback of giving rise to degraded beams when the excitation sources are remote from the focus of the reflector integrated between the parallel plates.
In beamformers of the type with constrained lenses, such as Ruze or Rotman lenses, the radiofrequency waves are constrained, that is to say guided, along a propagation path not corresponding to a natural optical path, in free space, such as defined by the Snell-Descartes laws. These beamformers can be synthesized so as to exhibit three or four different foci, thereby making it possible to obtain fewer aberrations and beams of better quality. However to control the delays of the radiofrequency waves propagating towards the lateral edges of the lens with respect to those propagating in an axial direction, towards the centre of the lens, these beamformers make it necessary for the radiofrequency waves to be tapped off along the internal contour of the lens by an array of various delay transmission lines. These delay transmission lines are distributed over the said internal contour of the lens and are connected to corresponding radiating elements whose ports define the external contour of the lens. The problem is that tapping off the radiofrequency waves disturbs the electromagnetic field which is sampled spatially and induces losses. Moreover, in order for the constrained-lens beamformer to be planar and for the lens to be completely integrated between the two parallel plates, it is necessary to add, over the path of the radiofrequency waves, delay transmission lines, for example rectangular waveguides, which induce a frequency dispersion and limit the bandwidth of the beamformer. To avoid frequency dispersion and to increase the bandwidth, in certain Rotman lenses, the transmission lines used are coaxial lines, but this requires the fashioning of a transition between the coaxial lines and the linear radiating aperture, and the structure of the beamformer is then not completely integrated. No solution currently exist for a beamformer of constrained lens type making it possible to circumvent the sampling of the radiofrequency waves.